1. Field of the Invention
The present invention pertains to seismology and, more particularly, seismology involving converted wave-form data.
2. Description of the Related Art
Reservoir fracture characterization has historically been a significant problem for production engineers and has resulted in higher than necessary production costs. In addition to borehole methods for identifying and classifying fractured reservoirs, surface seismic methods can provide important attributes for quantifying fractures over large spatial areas between wells. These attributes are related to the observed anisotropy in reflection amplitudes and travel times of compressional waves, also known as “P-waves,” and shear-waves, also known as “S-waves” that are commonly used in seismic surveys.
More particularly, there are generally two types of seismic waves used in seismology. The first type are the so-called “P-waves,” or compressional waves, in which the vibrations occur in the direction of propagation of the waves. The second type are the so-called “S-waves,” or shear-waves, in which the vibrations occur in a direction generally orthogonal to the direction of propagation of the waves. S-waves typically split into a fast and slow wave in birefringent, or anisotropic, media, and the amplitudes and travel times of both kinds of shear-waves may be used. Furthermore, as will be explained below, the incoming shear-waves may contain two components which are polarized (in terms of the direction of vibration) in two orthogonal directions, S1 (i.e., the fast shear S1 propagation direction) and S2 (i.e., the slow shear S2 propagation direction), and which are separated from each other by a time delay.
Many seismic surveys also employ “converted” waves. From ocean bottom or land multi-component surveys using a P-wave source, it is possible to obtain measurements of the S-waves converted in the earth. If the earth is isotropic with respect to the horizontal direction of wave motion, then a single S-wave arrival may be expected for each reflecting interface. If however, as is often the case, the earth behaves anisotropically with respect to the horizontal direction (for example, because a geological layer is polarized in a particular direction due to fracturing), two separate S-wave arrivals from each reflecting interface, arriving at different times, having propagated with different velocities, will be recorded. These are the fast (S1) and the slow (S2) S-waves. As previously mentioned, they are also characterized by having different polarization directions (i.e., directions of particle motion in the horizontal plane), which in most cases are considered to be approximately orthogonal to each other.
The shear-wave splitting phenomenon is illustrated in FIG. 1, which depicts a shear-wave arrival S that, at the start A of an anisotropic medium, splits into two separate shear-waves S1 and S2 having different polarization directions and propagating separately with differing velocities until the end B of the medium. If from the end B onwards the medium is supposed to be isotropic, the two polarized, split shear-waves will continue to travel separately but with the same velocity until they impinge upon the recording geophones. The amplitudes recorded on each of the horizontal components of the multicomponent geophone depend upon the orientations of the S1 and S2 directions relative to the X and Y directions.
FIG. 1 presents a simple graphical description of the principle of shear-wave birefringence, by only considering one anisotropic layer imbedded in an isotropic medium. However, in reality there are many reflecting boundaries that give rise to a number of shear arrivals polarized in the S1 and S2 directions. In addition, these S1 and S2 directions can change between the different anisotropic layers. In the applications considered here, the S1 and S2 polarization directions are assumed to be constant with depth, over the analyzing time window.
Thus, seismic surveying traditionally involves imparting acoustic waves from an acoustic source that propagate through subterranean geological formations and are reflected back to seismic sensors. The acoustic waves are typically P-waves and/or S-waves as discussed above, and sometimes converted waves. The seismic sensors are arrayed throughout the area being surveyed to receive the reflected waves. Seismologists frequently characterize the reflected waves as pure mode reflections, i.e., where the down-going and up-going legs of a reflection are of the same type (both P-wave or both S-wave), or converted mode reflections, i.e., P-wave on the downward leg and S-wave on the upward leg (PS-wave).
Pure mode reflections are symmetrical, whereas converted mode reflections are asymmetrical. FIG. 2A-FIG. 2D graphically illustrate this characteristic in a conceptual fashion. As shown in FIG. 2A-FIG. 2B, the propagation path of a pure mode reflection is symmetrical in that the response is the same for waves traveling in opposite directions. As shown in FIG. 2C-FIG. 2D, the propagation path of a converted mode reflection is asymmetrical, i.e., the response will differ depending on the direction of travel.
One consequence of their symmetry is that pure mode reflections are limited in their ability to quantify all the symmetry attributes of the medium. This is because the survey observes an average response of the down-going and up-going waves as they travel through the medium. The response is the same for waves traveling in opposite directions. As a result, these modes cannot alone distinguish between vertical fractures and dipping fractures in the geological formation.
One consequence of the asymmetry of a converted mode reflection's propagation is that they can, in principal, measure all the symmetry properties of a fractured medium. The split S-wave response will be different depending on the direction of travel. As a result, PS-waves have the ability to identify fracture dip, the orientation and direction of dip, in addition to other attributes provided by pure mode reflections. The symmetry properties of dipping fractures are not only important for characterizing the velocity structure during processing but more importantly they are important for planning horizontal drilling programs where wells are typically drilled normal to the fractures to maximize the drainage volume in the reservoir.
For symmetric P-wave modes a variety of techniques have been developed to characterize anisotropic media with a horizontal axis of symmetry using azimuthal velocity analysis and AVO/AVA (“Amplitude Variation with Offset/Azimuthal Velocity Analysis”) inversion. See Grechka, V. & Tsvankin, I., “3D Description of Moveout in Anisotropic Inhomogeneous Media,” 63 Geophysics 1079-92 (1998); Rüger, A., “P-Wave Reflection Coefficients for Transversely Isotropic Models With Vertical and Horizontal Axis of Symmetry,” 62 Geophysics 713-22 (1997). Anisotropic parameters, which can be inverted for fracture strike and density are calculated from elliptical velocity and AVA variations. See Hall, S., et al., “Fracture Characterization Using P-wave AVOA in 3-D OBS Data,” 70th Ann. Internat. Mtg.: Soc. of Expl. Geophys. 1409-12 (1999), Perez, M. A., et al., “Detection of Fracture Orientation Using Azimuthal Variation of P-Wave AVO Responses,” 64 Geophysics 1253-65 (1999).
Vertical fractures can be characterized when pure S-modes are acquired with two orthogonal horizontal shear-wave sources and receivers, respectively, as shown for land seismic data from Vacuum Field, N. Mex. See Roche, S. L. et al., “4-D, 3-C Seismic Study at Vacuum Field, N. Mex.,” SEG Expanded Abstracts 886-89 (1997); Angerer, E., et al., “Processing, Modeling, and Predicting Time-Lapse Effects of Over-Pressured Fluid Injection in a Fractured Reservoir,” _ Geophysical J. Int. _ (2001). Under the assumption that stacked data represent data traveling at a vertical ray-path, crack density, and orientation of vertical cracks can be determined in a layer stripping approach. De Vault, B., et al., “Multicomponent AVO Analysis at Vacuum Field, N. Mex., Part I: Theory and Data Processing,” 68th Ann. Internat. Mtg: Soc. of Expl. Geophys., 166-69 (1997), evaluated crack density of the same data using shear-wave AVO/AVA inversion. Both layer stripping and AVO inversion results correlate with the fault interpretation.
Converted mode reflections that sample numerous azimuths have the potential for fracture characterization by exploiting the effects of birefringence on the up-going S-waves. Potters, J. H. H. M., et al., “The 3D Shear Experiment Over the Natih Field in Oman: Reservoir Geology, Data Acquisition and Anisotropy Analysis,” 47 Geophy. Prosp. 637-62 (1999) demonstrated the importance of S-wave vibrator data for fracture characterization over the Natih field in Oman.
Although the above observations have provided important information with regard to fracture orientation and density, they are limited in their ability to quantify all the symmetry attributes of fractures. This is because, as noted above, pure modes, where the down-going and up-going legs of a reflection are the same (PP-wave or SS-wave), result in an averaged response. This response is the same for waves traveling in opposite directions; so pure modes cannot distinguish between vertical and dipping fractures.
PS-waves, which have only one S-wave leg (up-going), have also been used to measure anisotropic seismic attributes for fracture characterization. Ata, E. & Michelena, R. J., “Mapping Distribution of Fractures in a Reservior With P-S Converted Waves,” 14 The Leading Edge 664-676 (1995), used three 2-D lines centered over a well in Venezuela to quantify fracture information. Although the spatial coverage was sparse, azimuthal anisotropy appeared to be caused by two fracture systems. A small 3-D/3-C survey collected in the Wind River basin in Wyoming to calibrate a larger P-wave effort had some measure of success in characterizing fracture anisotropy. See Gaiser, J. E., “Applications for Vector Coordinate Systems of 3-D Converted-Wave Data,” 18 The Leading Edge 1290-1300 (1999); Grimm, R. E., et al., “Detection and Analysis of Naturally Fractured Gas Reservoise: Multiazimuth Seismic Surveys in the Wind River Basin, Wyoming,” 64 Geophysics 1277-92 (1999).
In addition, a 3-D/3-C survey collected in the Green River basin in Wyoming provided consistent PS-wave birefringence observations that correlated well with known faults and lineaments. See Gaiser, J. E. & Van Dok, R. R., “Analysis of PS-Wave Birefringence From a 3-D Land Survey for Fracture Characterization,” 63d EAGE Conf. and Tech. Exhibit, Amsterdam, Extended Abstract (2001). Marine PS-wave data also routinely show the presence of azimuthal anisotropy in the North Sea, Gaiser, J. E., “3-D PS-Wave Data: Unraveling Shear-Wave Birefreingence for Fracture Detection,” 62nd EAGE Conf. and Tech. Exhibit, Glasgow, Extended Abstract (2000); Probert, T., et al., “A Case Study of Azimuthal Anisotropy Analysis From a North Sea 3D 4C Project,” SEG/EAGE Summer Research Workshop, Boise, Id. (2000), and in the Gulf of Mexico, Gaiser, J. E., “Advantages of 3-D PS-Wave Data to Unravel S-Wave Birefrengence for Fracture Detection,” 70th Ann. Int'l SEG Mtg., Expanded Abstact, 1202-04 (2000); Spitz, S. et al., “Reservoir Monitoring Using Multicomponent Seismic: Processing the Teal South 4D-4C,” SEG/EAGE Summer Research Summer Workshop, Boise, Id. (2000), and this anisotropy is believed to be in response to fracturing.
However, these converted-wave surveys have also only considered anisotropy related to vertical fracture systems. For example, Gaiser and Van Dok, (2001), supra, used a four-component Alford rotation, see Alford, R. M., “Shear Data in the Presence of Azimuthal Anisotropy: Dilley, Tex.,” 56th Ann. Internat. Mtg., Soc. Expl. Geophys., Houston, Expanded Abstracts (1986), and layer stripping method, see Winterstein, D. F., & Meadows, M. A., “Shear-Wave Polarizations and Subsurface Stress Directions at Lost Hills Field,” 56 Geophysics 1331-38 (1991), to infer the density and orientation of vertical fractures.
As mentioned above, converted wave reflections are asymmetric where only the up-going wave consists of split S-waves. These S-wave paths have the same property as the down-going or transmitted S-waves in vertical seismic profiles (“VSPs”), i.e., it is a one-way path. Winterstein & Meadows (1991), supra, have shown how these can be used to measure the orientation and time delay between split S-waves for characterizing azimuthal anisotropy and fractures. Horne, S. A., et al., “Fracture Characterization From Near-Offset VSP Inversion,” 45 Geophysical Prospecting 141-64 (1997), have extended these techniques to measure the symmetry properties of a dipping fracture set using appropriate VSP data. Also, Grechka, V. & Tsvankin, I., “Inversion of Azimuthally Dependent NMO Velocity in Transversely Isotropic Media With a Tilted Axis of Symmetry,” 65 Geophysics 232-46 (2000) have devised a fracture-characterization procedure that estimates all background and fracture parameters of dipping fractures. However, the method uses long wavelength vertical and NMO velocities of P-wave and two S-waves (or converted waves) reflected from a horizontal interface. In practice, inverting these velocities for interval properties results in large uncertainties.
Some of these techniques have made their way into the patent literature. Consider U.S. Pat. No. 6,292,754, entitled “Vector Recomposition of Seismic 3-D Converted-Wave Data,” and issued Sep. 18, 2001, to BP Corporation North America Inc. as assignee of the inventor Leon Thomsen. The '754 patent discloses a method of seismic processing of multi-component converted wave 2-D and 3-D seismic data, wherein the seismic traces in each CCP gather may have been acquired at a variety of different source-receiver azimuths. However, the technique is applied only on prestacked data. Furthermore, the data is not organized into orthogonal azimuth distribution. Instead, all azimuths are considered and used within the process, which average the time-shifts or attributes to be derived. This introduces inaccuracies in the analysis predicated on the resulting data.
Consider also International Application WO0136999 A2, entitled “Determination of the Fast and Slow Shear-Wave Polarization Directions,” filed Oct. 20, 2000, published May 25, 2001, and claiming a priority date of Nov. 16, 1999. Portions of this application are excerpted above relative to FIG. 1. The '999 application proposes one way of obtaining attributes from converted wave data using only one set of azimuth data (i.e., one direction) which means two-traces with the same azimuth (due to symmetry). This also introduces some inaccuracies in the analysis predicated on the resulting data.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.